The present invention relates to papermaking machines in general, and more particularly to improvements in apparatus for expelling moisture from running webs or sheets of fibrous material. Still more particularly, the invention relates to improvements in apparatus wherein a running web of paper or the like is dried during travel along and with the periphery of a rotary heating member, especially a Yankee cylinder.
As a rule, the cylindrical shell of a Yankee cylinder consists of steel or cast iron. The interior of the shell is heated by steam which is maintained at an elevated pressure in order to insure a very high heat energy output, i.e., a pronounced drying action upon the running web during travel along that portion of the path wherein the web is maintained in contact with the periphery of the Yankee cylinder. The shell of the cylinder plays a very important role because its material effects the transfer of heat energy from the interior of the cylinder to the running web. Therefore, manufacturers of Yankee cylinders and analogous rotary drying devices strive to reduce the thickness of the cylinder shell. However, the thickness of the shell cannot be reduced at will since a Yankee cylinder is often quite large and bulky so that its shell must be capable of withstanding very pronounced deforming stresses. Moreover, the cylinder must withstand the pressure of the confined heating medium. Furthermore, and since the thickness of the shell cannot be reduced below a minimum thickness which is still quite substantial, this automatically eliminates the majority of materials having a high thermal conductivity. Such materials include copper and other metals or alloys of equally satisfactory thermal conductivity. Consequently, and as mentioned above, the shells of presently known Yankee cylinders invariably consist of a ferrous metal, primarily cast iron or steel.